Metal-organic frameworks for Xe/Kr separation

ABSTRACT

Metal-organic framework (MOF) materials are provided and are selectively adsorbent to xenon (Xe) over another noble gas such as krypton (Kr) and/or argon (Ar) as a result of having framework voids (pores) sized to this end. MOF materials having pores that are capable of accommodating a Xe atom but have a small enough pore size to receive no more than one Xe atom are desired to preferentially adsorb Xe over Kr in a multi-component (Xe—Kr mixture) adsorption method. The MOF material has 20% or more, preferably 40% or more, of the total pore volume in a pore size range of 0.45-0.75 nm which can selectively adsorb Xe over Kr in a multi-component Xe—Kr mixture over a pressure range of 0.01 to 1.0 MPa.

RELATED APPLICATION

This application claims benefits and priority of U.S. provisionalapplication Ser. No. 61/401,816 filed Aug. 19, 2010, the entiredisclosure of which is incorporated herein by reference.

CONTRACTUAL ORIGIN OF THE INVENTION

This invention was made with government support under Grant No.DE-FG02-03ER15457 awarded by the Department of Energy. The governmenthas certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates to separation of noble gases such as xenonand krypton using certain metal-organic framework (MOFs) materials.

BACKGROUND OF THE INVENTION

Separating xenon from krypton is an industrially important problem.Xenon (Xe) and krypton (Kr) are used in fluorescent light bulbs, andcurrent technology produces these gases from the cryogenic distillationof air, in which these noble gases are present in small concentrations(1.14 ppmv Kr, 0.086 ppmv Xe). Both xenon and krypton separate into theoxygen-rich stream after distillation, and these gases are concentratedand purified to produce an 80/20 molar mixture of krypton to xenon.¹This final mixture typically undergoes further cryogenic distillation toproduce pure krypton and pure xenon. Distillation is an energy-intensiveprocess, and separation of these gases by selective adsorption near roomtemperature would be much more energy efficient. Additionally,separating krypton from xenon is an important step in removingradioactive krypton-85 during treatment of spent nuclear fuel.² However,even after cryogenic distillation, trace levels of radioactive kryptonin the xenon-rich phase are too high to permit further use.² Ifadsorbents could reduce krypton-85 concentrations in the xenon-richphase to permissible levels, there could be an entirely new supplysource of xenon for industrial use. Thus, there is a strong need todevelop adsorbent materials for this separation to reduce energyconsumption and to reuse byproducts of consumed nuclear fuel.

There are several examples in the literature where zeolites have beentested for Xe/Kr separation. Previous research has shown NaX zeolite tobe a selective adsorbent for xenon over krypton with a selectivity ofabout 6 with krypton concentrations ranging from 1 to 10,000 ppm.²Jameson et al.³ showed that NaA zeolite had a selectivity ofapproximately 4 for binary mixtures of xenon and krypton at 300 Kbetween 1 and 10 bar. They also used molecular simulations to show thatideal adsorbed solution theory (IAST) could accurately predict theselectivities and mixture behavior from the single-component isotherms.

Metal-organic frameworks,⁴⁻⁶ or MOFs, are a new class of nanoporousmaterials. Composed of organic linkers and metal corners, thesematerials self-assemble in solution to form stable, crystallineframeworks. Coordination bonds between oxygen and nitrogen atoms withmetal centers allow for a variety of topologies, and choice of theorganic linker allows one to tailor pore sizes and environments forparticular applications. As a result, these materials have garnered muchattention for hydrogen storage,⁷⁻⁹ separations,^(10,11) andcatalysis.¹²⁻¹⁴

A number of groups have investigated MOFs for separation of other gases.For example, Bae et al.¹⁵ used both experiments and simulation to show amixed-ligand MOF effectively separates carbon dioxide from methane. Baeet al.¹⁶ also showed that exchanging fluorinated-methylpyridine into aMOF could substantially increase the selectivity of carbon dioxide overnitrogen due to the increased polar environment. Pan et al.¹⁷synthesized a microporous MOF with 1D hydrophobic microchannels anddemonstrated its ability to separate n-butane from other n-alkanes andolefins. Hartmann et al.¹⁸ showed that isobutene can be separated fromisobutane using HKUST-1 in a breakthrough system. Yang et al.^(19,20)used molecular simulations to predict that HKUST-1 is a promisingcandidate for separation of carbon dioxide from both air andmethane/hydrogen mixtures.

To date, there are a few publications that report the investigation ofXe/Kr separation using MOFs. Mueller et al.²¹ measured noble gasadsorption in IRMOF-1 and noticed significantly higher adsorption forthe heavier gases, namely xenon and krypton, in MOF-filled containersrelative to containers without MOF material. Building on these results,they built a breakthrough system filled with HKUST-1 and showed that a94/6 molar mixture of krypton/xenon could be purified to over 99%krypton and less than 50 ppm xenon. Greathouse et al.²² recentlysimulated noble gas adsorption in IRMOF-1. They predicted that IRMOF-1has a selectivity of about 2.5-3 for Xe over Kr at 298 K and pressuresof both 1 and 10 bar.

SUMMARY OF THE INVENTION

The present invention envisions a method of separating a particularnoble gas in a gas mixture by contacting the gas mixture with anadsorbent material comprising a metal-organic framework (MOF) materialhaving framework pores that are sized to receive no more than one atomof the particular noble gas for selectively adsorbing the particularnoble gas from the gas mixture. MOF materials having a relatively highpercentage of pores (percentage of total pore volume) that are capableof accommodating the noble gas atom but that have a small enough poresize to receive no more than one such atom are desired to preferentiallyadsorb the particular noble gas over one or more other noble gases in amulti-component mixture adsorption method embodiment. However, the poresize cannot be so small that it significantly limits overall gas uptake(capacity), which is undesirable. The present invention thus envisionsMOF adsorbent materials for separation of a particular noble gas and oneor more other noble gases in a gas mixture.

In an illustrative embodiment of the invention, the present inventionprovides metal-organic framework (MOF) materials that are selectivelyadsorbent to xenon (Xe) over another noble gas, such as for examplekrypton (Kr) and/or argon (Ar), as a result of having a framework voids(pores) sized to this end. MOF materials having 20% or more, preferably40% or more, of the total pore volume, capable of accommodating a Xeatom but having a small enough pore size to receive no more than one Xeatom are desired to preferentially adsorb Xe over Kr in amulti-component (Xe—Kr gas mixture) adsorption method. The presentinvention thus envisions MOF adsorbent materials for separation of xenon(Xe) and one or more other noble gases.

An illustrative Xe-selective MOF material includes characteristicmultiple pore size categories within its particular framework wherein20% or more, preferably 40% or more, of the total pore volume has a sizein the range of 0.45-0.75 nm, which compares to the Lennard-Jonesdiameters of 0.4100 nm and 0.3636 nm for Xe and Kr, respectively. SuchMOF materials can selectively adsorb Xe over another noble gas, such asKr, in a multi-component gas mixture over a pressure range of 0.01 to1.0 MPa.

In a particular illustrative embodiment of the present invention, aXe-selective adsorbent material having a chemical formula unitrepresented by Cu₂(3,3′,5,5′-biphenyltetracarboxylate) and having theNbO topology has been identified, tested and determined to exhibitincreased xenon selectivity over a wider pressure range of 0.01 to 1.0MPa compared to other MOFs materials. The material was found to exhibita selectively of about 9 to about 11 in the pressure range of 0.1 to 1.0MPa.

Advantages and detailed features of the present invention will becomemore apparent from the following detailed description taken with thefollowing drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1: Single-component (open symbols) and mixture (filled symbols)isotherms for Xe and Kr adsorption in UMCM-1 at 273 K.

FIG. 2: Single-component (open symbols) and mixture (filled symbols)isotherms for Xe and Kr adsorption in HKUST-1 at 273 K. At low pressures(approximately 0.1 bar), the xenon selectivity is nearly 17, which ismuch higher than the selectivities estimated from the single-componentisotherms.

FIG. 3: Heats of adsorption versus loading for Kr (circles) and Xe(squares) single-component adsorption in HKUST-1 at 273 K, as well asaveraged sorbate-MOF interaction energies. At low loading, adsorption inthe small octahedral pockets leads to high heats of adsorption.

FIG. 4: Mixture isotherms from GCMC simulations (filled symbols) andisotherms predicted by IAST based on single-component isotherms (opensymbols) for Xe and Kr adsorption in HKUST-1 at 273 K. IAST does notcorrectly predict Xe and Kr adsorption of the mixture results since itcannot correctly capture competitive adsorption in the octahedralpockets.

FIG. 5: Single-component (open symbols) and mixture (filled symbols)isotherms for Xe and Kr adsorption in MOF-505 at 273 K. Unlike HKUST-1,xenon selectivity remains elevated even at higher pressures, making it apromising candidate material for Xe/Kr separation.

FIG. 6: Single-component (open symbols) and mixture (filled symbols)isotherms for Xe and Kr adsorption in NOTT-101 at 273 K. The extraphenyl moiety in the linker compared to MOF-505 makes the pore size toolarge and leads to lower selectivities relative to those predicted forMOF-505.

FIG. 7: Single-component (open symbols) and mixture (filled symbols)isotherms for Xe and Kr adsorption in Pd-MOF at 273 K.

FIG. 8: Breakthrough curves for the separation of Xe/Kr mixtures(Xe:Kr=20:80 vol. %) over MOF-505 pellets at room temperature. The totalflow is 10 ml/min and the pressure is 1 bar.

FIG. 9: Pore size distribution for IRMOF-1 where [−dP(r)/r] on thevertical axis means the fraction of pores having radius r and Radius(nm) on the horizontal axis corresponds to the radius of frameworkpores.

FIG. 10: Single component (open symbols) and mixture (filled symbols)isotherms for Xe and Kr adsorption in IRMOF-1 at 273 K. The Xe/Krselectivity remains relatively constant at about 3.5-4 over thispressure range.

FIG. 11: Mixture isotherms (filled symbols) and those predicted by IAST(open symbols) for Xe and Kr adsorption in IRMOF-1 at 273 K. IASTreproduces the mixture results very well over the pressure rangeinvestigated.

FIG. 12: Pore size distribution for UMCM-1.

FIG. 13: Mixture isotherms (filled symbols) and those predicted by IAST(open symbols) for Xe and Kr adsorption in UMCM-1 at 273 K.

FIG. 14: Pore size distribution for ZIF-8.

FIG. 15: Single component (open symbols) and mixture (filled symbols)isotherms for Xe and Kr adsorption in ZIF-8 at 273 K.

FIG. 16: Heats of adsorption versus loading for Kr (circles) and Xe(squares) single-component adsorption in ZIF-8 at 273 K as well as thesorbate-MOF contribution. The presence of attractive sorbate-sorbateinteractions at higher loading is responsible for the gradual increasein isosteric heats.

FIG. 17: Pore size distribution for HKUST-1.

FIG. 18: Pore size distribution for MOF-505.

FIG. 19: Pore size distribution for NOTT-101.

FIG. 20: Pore size distribution for NOTT-108.

FIG. 21: Single-component (open symbols) and mixture (filled symbols)isotherms of Xe and Kr adsorption in NOTT-108 at 273 K.

FIG. 22: Pore size distribution for Pd-MOF.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides metal-organic framework (MOP) materialsthat are selectively adsorbent to xenon (Xe) over another noble gas,such as for example krypton (Kr) and/or argon (Ar), as a result ofhaving a framework voids (pores) sized to this end. MOF materials having20% or more, preferably 40% or more, of the total pore volume capable ofaccommodating a Xe atom but having a small enough pore size to receiveno more than one Xe atom are desired to preferentially adsorb Xe over Krin a multi-component (Xe—Kr gas mixture) adsorption method. The presentinvention thus envisions particular MOF adsorbent materials forseparation of xenon (Xe) and one or more other noble gases.

For purposes of illustration and not limitation, a number of MOFs withdifferent pore sizes, linkers, metal atoms, and topologies were chosenin order to sample a variety of MOF properties and gain insight intowhich characteristics are desired for separation of Xe and one or moreother noble gases, such as for example Kr and/or Ar. The selected MOFsare IRMOF-1,²³ UMCM-1,²⁴ ZIF-8,²⁵ HKUST-1,²⁶ MOF-505,²⁷NOTT-101,²⁸NOTT-108,²⁸ and Pd-MOF.²⁹ These MOFs are described in detail in thereferences noted by superscripts, the teachings of these referencesbeing incorporated herein by reference to this end. These MOF's werescreened for xenon/krypton separation using grand canonical Monte Carlo(GCMC) simulations as described below.

The MOF designated MOF-505 was made for actual testing as describedbelow after conduct of the screening.

MOF-505 Crystal Synthesis:

Make Solution A: 15 ml DMF with 8 drops of 15% HBF₄

Take:: 15 mg organic ligand (0.045 mmol)

-   -   65 mg Cu(NO₃)₂*2.5H₂O (0.28 mmol)    -   2 ml of Solution A    -   combine in 2 dram vials sonnicated

heat in oven 90 degrees C. for 1 day yields light blue crystallinepowder

The chemical formula unit for each MOF is listed below in Table 1:

TABLE 1 Chemical formula Unit Material Formula Unit IRMOF-1Zn₄O(1,4-benzenedicarboxylate)₃ UMCM-1Zn₄O(1,4-benzenedicarboxylate)(1,3,5-tris(4-benzenecarboxylate)benzene)_(4/3) ZIF-8 Zn(2-methylimidazolate)₂(Sodalite topology) HKUST-1 Cu₃(1,3,5-benzenetricarboxylate)₂ MOF-505Cu₂(3,3′,5,5′-biphenyltetracarboxylate) (NbO topology) NOTT-101Cu₂(3,3″,5,5″-triphenyltetracarboxylate) NOTT-108Cu₂(2′,3′,5′,6′-tetrafluoro-3,3″,5,5″-triphenyltetracarboxylate) Pd-MOFPd(2-hydroxypyrimidinolate)₂

Applicants calculated the pore size distribution for each of these MOFs.The pore size distribution of a MOF is calculated by randomly selectinga point within the unit cell volume (that does not overlap withframework atoms) and calculating the largest sphere that can fit withinthe MOF that includes that particular point. This process is repeateduntil the entire unit cell volume has been sampled sufficiently. Thedistribution is then plotted as a function of the sphere radius. Adetailed description of this process is provided by Gelb L D, Gubbins, KE, Pore size distribution in porous glasses: A computer simulationstudy, Langmuir, 1999; 15; 305-308, the teachings of which areincorporated herein by reference.

The results are summarized in Table 2.

TABLE 2 Pore diameters of all MOFs investigated estimated from pore sizedistribution calculations. MOF Pore Sizes (nm) IRMOF-1 1.12 1.45 UMCM-11.03 1.39 2.33 ZIF-8 1.05 HKUST-1 0.50 1.06 1.24 MOF-505 0.48 0.71 0.95NOTT-101 0.50 0.96 1.05 NOTT-108 0.45 0.89 1.05 Pd-MOF 0.22 0.49 0.58

IRMOF-1 is composed of Zn₄O corners and benzenedicarboxylate (BDC)linkers and has large pore diameters of L12 and 1.45 nm. FIG. 9. UMCM-1also has Zn₄O corners but has two organic linkers: BDC and1,3,5-tris(4-carboxyphenyl)benzene (BTB). This MOF contains bothmicroporous and mesoporous cavities or pores, FIG. 12.Zeolitic-imidazolate-framework #8 (ZIF-8) contains tetrahedral Zn²⁺atoms coordinated to methylimidazolate linkers in a sodalite-typeframework. Its main cavity or pore spans 1.05 nm across, FIG. 14.HKUST-1 is made of copper paddlewheels with benzenetricarboxylate (BTC)linkers, which form both small and large pockets or pores. The smallerpockets or pores have diameters of 0.50 nm, while the larger pockets orpores have diameters of 1.06 and 1.24 nm, FIG. 17. The window into thesesmall pockets or pores is approximately 0.46 nm, and previousexperimental work using ¹²⁹Xe NMR spectroscopy³⁰ has demonstrated thatthese sites are accessible to xenon. Based on the size of krypton andxenon, whose Lennard-Jones diameters are 0.3636 and 0.4100 nm.respectively, these pockets or pores should be accessible as adsorptionsites. However, only about 12% of the total pore volume has the smallsize (0.50 nm pore diameter). MOF-505 is composed of Cu²⁺ paddlewheelscoordinated to biphenyltetracarboxylate linkers and has characteristicpores of 0.48, 0.71, and 0.95 nm diameter where about 46% of the totalpore volume is in the range of 0.45-0.75 nm, FIG. 18. Similar toMOF-505, NOTT-101 uses Cu²⁺ paddlewheels with triphenyltetracarboxylatelinkers and results in slightly larger pores of 0.50, 0.96, and 1.05 nmdiameter, FIG. 19. NOTT-108 is identical to NOTT-101 with the exceptionof four fluorine atoms in the place of hydrogens on the middle phenylmoiety of the triphenyl linker. The pore distribution of NOTT-108 isshown in FIG. 20. Since the NOTT-101 crystal structure was not publishedin the original paper, applicants constructed a NOTT-101 structure byusing the NOTT-108 structure and manually changing fluorine atoms tohydrogen atoms and adjusting the carbon-fluorine bond lengths. Pd-MOFcontains Pd²⁺ cations bonded to 2-hydroxypyrimidinolate linkers in asodalite topology. The coordinated Pd atoms exist in a square planarconfiguration, and the MOF has pore diameters of approximately 0.22,0.49, and 0.58 nm, FIG. 22.

Grand canonical Monte Carlo (GCMC) calculations were performed tosimulate adsorption in these MOFs.^(31,32) A total of 50.000equilibration cycles and 250,000 production cycles were used for eachsimulation. One cycle consists of N moves, where N is the number ofmolecules (minimum of 20 moves). Insertion, deletion, translation, andidentity change moves (e.g., change Xe to Kr) were considered. Bydividing the production run into 5 independent blocks and calculatingthe standard deviation of the block averages, an average error of 1.3%in the loading is estimated at the 95% confidence interval. Usingpropagation of error, the selectivities reported have estimated errorsof 1.8% at the 95% confidence interval. Single-component and mixtureisotherms were simulated for each MOF. The mixture isotherms had a fixed80/20 molar composition of krypton to xenon in the gas phase to berepresentative of an industrial mixture. Fugacities were calculatedusing the Peng-Robinson equation of state. Framework atoms wereconsidered fixed at their crystallographic coordinates. Thisapproximation of a rigid framework has been shown to be a reasonablestrategy for screening adsorption in MOFs.²² A 12-6 Lennard-Jonespotential was used to describe sorbate-framework interactions. For theMOF atoms, van der Waals parameters were taken from the DREIDING³³ forcefield and, if not available, from the UFF³⁴ forcefield. This choice offorcefield has been effective in past studies of hydrogen and methaneadsorption in IRMOF-1,^(35,36) as well as CO₂ adsorption in a variety ofMOFs.^(37,38) A cutoff of 1.2 nm was used for the van der Waalsinteractions. Krypton³⁹ and xenon⁴⁰ parameters were obtained from theliterature. Lorentz-Berthelot mixing rules were used for thegas/framework interactions. No electrostatic charges were considered.Selectivities from the mixture isotherms were calculated with thestandard definition:Selectivity=(x _(Xe) /y _(Xe))/(x _(Kr) /y _(Kr))where x_(i) is the adsorbed phase mole fraction of component i and y_(i)is the gas phase mole fraction of component i. Additionally, in somecases we crudely predicted selectivities of mixture adsorption from thesingle-component isotherms by calculating the ratio of the amount ofadsorbed Xe at a given pressure to the amount of adsorbed Kr at the samepressure. All simulations were performed at 273 K. All data reported areexcess adsorption isotherms, which can be calculated using absoluteadsorption values, pore volume, and bulk fluid density.⁴¹ Also, IASTcalculations were performed to determine whether single-componentisotherms could be used to accurately predict the results from fullmixture simulations.

RESULTS

IRMOF-1 (also known as MOF-5) is probably the most studied MOF to date.The results for xenon and krypton adsorption in IRMOF-1 are comparableto the previous results from Greathouse et al.²² (FIG. 10).Selectivities of about 3.5 to 4 for xenon over krypton are predicted,and this selectivity changes very little as a function of pressure.Adsorption in UMCM-1 also displays xenon selectivities of about 3.5 to4, as shown in FIG. 1. Although the capacity of UMCM-1 is by far thelargest of all MOFs investigated here (about 15 and 25 mol/kg insingle-component isotherms for Kr and Xe, respectively), the xenonselectivity does not represent a significant improvement relative topreviously reported values for zeolites. Density plots of mixtureadsorption in UMCM-1 at 6 bar and at 273 K for Kr and Xe showed that themajority of adsorption is near the Zn₄O corners and along the organiclinkers. Density plots at 0.6 MPa suggest that xenon and krypton atomsadsorb next to organic linkers once stronger sites near the corners arefilled, which is a mechanism that was previously found for othermolecules in IRMOFs.⁴² Note that before the simulation, the unit cellwas divided into 150×150×150 voxels. After each cycle, the number ofadsorbed molecules is counted, and their corresponding voxel values areupdated accordingly. After normalization with respect to the highestoccurring value in the histogram, the probability of finding an adsorbedmolecule is represented). Additionally, crudely estimating selectivityof mixture adsorption from the single-component data allows one toobtain a good estimate of the xenon selectivity calculated explicitlyfrom the mixture simulations. Adsorption in both IRMOF-1 and UMCM-1follows IAST (FIGS. 11 and 13). Given the relatively modestselectivities in these large-pore MOFs, applicants thought it would bebeneficial to study MOFs with smaller pore sizes where there arestronger adsorption sites to enhance the selectivity.

This study is borne out by the mixture isotherms in ZIF-8 that show amaximum xenon selectivity of about 7 (FIG. 15). Its smaller pores yieldstronger adsorption sites, which favor Xe binding and lead to anincrease in selectivity. FIG. 16 shows the heats of adsorption forsingle components in ZIF-8. The heats of adsorption rise for both Xe andKr with increasing loading, due to the increasing importance ofsorbate-sorbate interactions. While the pores of ZIF-8 are smaller thanthose of IRMOF-1 and UMCM-1, the pore diameters of ZIF-8 are still largerelative to the size of xenon and krypton (0.4100 and 0.3636 nm,respectively). This void space lacks adsorption sites that are strongenough to enhance xenon selectivity dramatically.

FIG. 2 shows the calculated isotherms for HKUST-1, which was previouslytested experimentally as an adsorbent material for Xe/Kr separation.²¹Although binary adsorption selectivities naively estimated from thesingle-component isotherms are between 3 and 4 up to 0.1 MPa, mixtureadsorption results predict much higher xenon selectivities: nearly 17 at0.01 MPa and 8 at 0.1 MPa. Density plots revealed the cause of thislarge selectivity at low coverage. In single-component adsorption, bothKr and Xe prefer to adsorb in the small octahedral pockets since theyare the strongest adsorption sites in the MOF. These pockets are sosmall that only one atom of Xe or Kr can fit inside. However, when abinary mixture is present, xenon and krypton directly compete for thesestrong binding sites. At low loading, heats of adsorption for kryptonand xenon have values of 21 and 32 kJ/mol, respectively, whichrepresents a significant increase relative to their values at marginallyhigher loading (FIG. 3). Due to stronger van der Waals interactions,xenon preferentially occupies this octahedral pocket and preventskrypton from adsorbing. Instead, krypton adsorbs near the pocketopenings. Adsorption in HKUST-1 leads to significant deviation fromIAST, which predicts xenon selectivities between 4 and 5 as shown inFIG. 4. This deviation is significant at low pressure, which is incontrast to most adsorption systems, where deviations from ideality aremore common at higher loadings.

The phenomenon of high selectivity in very small pores is applied topicking candidate MOFs for Xe/Kr separation. That is, MOFs withadsorption sites that are large enough to accommodate a Xe atom butsmall enough to fit only one atom are attractive candidates. Forexample, while HKUST-1 showed preferential adsorption sites, the xenonselectivity drops considerably from 17 at 0.01 MPa to 8 at 0.1 MPa andnearly approaches that predicted by IAST around 1.0-1.5 MPa because ofincreased adsorption in the larger pores. These results show thatalthough adsorption in the octahedral pockets is highly non-ideal,adsorption in the larger pores is ideal and increasingly contributes tothe overall xenon selectivity as the pressure and gas loading areincreased. Therefore, applicants examined other MOFs to identify thosewith smaller pores that also impart non-ideal adsorption and maintainenhanced xenon selectivity over a wider pressure range.

One attractive candidate pursuant to the invention was MOF-505, whichhas smaller average pore sizes than HKUST-1. The simulation results forthis MOF are shown in FIG. 5. The steepness of the single-componentisotherms indicates that small pores are present and that adsorbedspecies have high heats of adsorption. Density plots revealed adsorptionof xenon and krypton near the pore openings in the MOF. Compared toHKUST-1, MOF-505 has smaller cavity or pore sizes, Table 1 and FIG. 18,which allow the xenon selectivity to remain elevated over a widepressure range (˜9 at 1 MPa).

Referring to Table 1, MOF-505 has two out of three categories of pores(cavities) that allow Xe selectively by virtue of accommodating only asingle Xe atom in each pore. In particular, MOF-505 material includestwo categories of pores that have a pore size in the range of 0.45-0.75nm; namely, pore sizes of 0.48 and 0.71 nm). From FIG. 18, it isapparent that about 46% of the total pore volume of MOF-505 is in thisrange in contrast to FIG. 17 for the HKUST-1 material, which has onlyabout 12% of thee total pore volume in this range. Such MOF materials asMOF-505 can selectively adsorb Xe over Kr in a multi-component Xe—Krmixture over a pressure range of 0.01 to 1.0 MPa.

Since typical pressure swing adsorption processes are run between 0.1and 0.5 MPa, the selectivities of MOF-505 of about 10 to 11 in thispressure range are superior to those of HKUST-1 (about 6 to 8), makingMOF-505 a more attractive MOF for Xe/Kr separation.

In order to explore whether the enhanced selectivity of MOF-505 was dueto pore size and not to framework topology, NOTT-101 and NOTT-108 weretested. These MOFs have the same topology as MOF-505, but are composedof slightly longer triphenyl linkers. FIG. 6 shows that the resultsfollow a similar trend to those for HKUST-1, where a maximum selectivityof about 10 occurs at 0.01 MPa for the mixture isotherms and thendecreases significantly with increasing pressure and gas loading (downto 5 at 3 MPa). The pore sizes of NOTT-101 are too large to ensurenon-ideal adsorption at higher loadings. The results. FIG. 21, forNOTT-108 can be interpreted similarly and are not significantlydifferent from those for NOTT-101 despite the presence of fluorine atomson the organic linkers.

Finally, applicants investigated Pd-MOF in order to test MOFs with evensmaller pores. FIG. 7 displays the simulation results. The pores are sosmall that the xenon selectivity is very high, remaining near 18 or 19for the entire pressure range from 0.01 to 3 MPa. Density plots revealedthat Xe and Kr can only adsorb in the larger cavities of the MOF and notin the octahedral pockets, which are too small. Xenon dominates theadsorption at all pressures and prevents krypton from adsorbing, leadingto the high selectivity. However, the adsorption capacities (up-take) ofboth Xe and Kr for Pd-MOF are significantly lower than those of theother MOFs. In fact, Pd-MOF had the lowest void fraction (0.348) of allMOFs investigated (void fractions of IRMOF-1=0.814; UMCM-1=0.871;ZIF-8=0.495; HKUST-1=0.746; MOF-505=0.741; NOTT-101=0.775; andNOTT-108=0.743). These results offer a preliminary estimate to themaximum xenon selectivity using competitive adsorption sites in MOFs.

EXAMPLE

Breakthrough measurements were performed on MOF-505 material synthesizedby the applicant. For example, 390 mg of MOF-505 pellets of pellet sizeof 600˜1000 μm were packed into a stainless steel column with a lengthof 12 cm and an internal diameter of 0.46 cm, and the remaining volumein the column was filled by glass wool. Helium gas was used to initiallypurge the system. At a certain time (t=0), a mixture of containing 80volume % Kr and 20 volume % Xe was introduced into the column at a flowrate of 10 ml/min. The flow rates of all gases were regulated by massflow controllers, and the effluent gas stream from the column wasmonitored by mass spectroscopy (MS).

The breakthrough result is shown in FIG. 8. Kr gas elutes rapidly fromthe column, whereas Xe is strongly retained. After an initial periodduring which both Xe and Kr are fully adsorbed, pure Kr elutes from thecolumn. The effluent Kr concentration slightly exceeds the feedconcentration because the more strongly retained Xe molecules displacesome of the adsorbed Kr molecules. When the breakthrough of Xe at thecolumn outlet starts, the effluent concentrations of the componentsevolve toward the feed concentration level as the column becomessaturated. The result of FIG. 8 demonstrates that MOF-505 has a highselectivity for Xe over Kr.

CONCLUSIONS

Applicants performed GCMC simulations of both single component andmixture adsorption of Xe and Kr in a variety of MOFs. The results aresummarized in Table 3 below. Pd-MOF is predicted to have the largestselectivity, and the high selectivity is maintained across a wide rangeof pressures in this material. Large pore materials are not desirablefor efficient Xe/Kr separation. Both IRMOF-1 and UMCM-1 show low xenonselectivities of about 4 and follow ideal adsorption. To enhanceselectivity, small pores or pockets are needed to preferably bind Xeinstead of Kr and introduce non-ideality to mixture adsorption. HKUST-1has a high adsorptive selectivity at low loading due to its smallpockets where Xe atoms adsorb with higher heats of adsorption than Kr.However, the selectivities in HKUST-1 drop off quickly at higherpressure due to the presence of large cavities or pores, which arefilled after the small octahedral pockets, demonstrating that the bestMOFs for Xe/Kr separation should have relatively high percentages ofsmall cavity sizes such as 20% or more, preferably 40% or more, of smallcavity sizes (e.g. 0.45-0.75 nm diameter).

MOF-505 has three types of pores with two out of three (about 46% oftotal pore volume) being relatively small in the range of 0.48 to 0.71nm diameter. This MOF maintains its elevated xenon selectivities over alarge pressure range as shown in the Example above.

NOTT-101 and NOTT-108 share the same topology with MOF-505 but havepores that are too large for efficient Xe/Kr separation.

TABLE 3 Xe/Kr selectivity for all MOFs investigated at pressures of 0.1bar, 1 bar, and 10 bar as predicted from mixture simulations at 273 K.Xe/Kr Selectivity MOF 0.1 bar 1 bar 10 bar IRMOF-1 3.6 3.7 4.1 UMCM-13.5 3.6 3.7 ZIF-8 6.5 6.8 5.6 HKUST-1 16.8 8.1 5.6 MOF-505 9.4 11.2 8.9NOTT-101 9.6 7.2 5.9 NOTT-108 11.0 7.7 6.1 Pd-MOF 19.4 19.4 18.0

Practice of the present invention may replace cryogenic distillation asthe preferred method for separating noble gases. The MOF adsorbentmaterials could be used as adsorbents in a pressure swing adsorptionprocess, which is much less energy intensive than distillation.Additionally, the enhanced selectivities obtained with MOFs compared tozeolites may allow for the treatment of spent nuclear fuel and increasethe industrial supply of xenon. Currently, high levels of radioactivekrypton-85 prevent these waste gases from further use.

Although the invention has been described in connection with certainembodiment, those skilled in the art will appreciate that changes andmodifications can made therein without departing from the spirit andscope of the invention as set forth in the appended claims.

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We claim:
 1. A method of separating Xe and another noble gas in a gasmixture, comprising contacting the gas mixture with an adsorbentmaterial comprising a metal-organic framework (MOF) material wherein theMOF material has 20% or more of the total pore volume of a size toreceive no more than one Xe atom for selectively adsorbing Xe from thegas mixture; wherein the MOF material has carboxylate linkers andwherein the MOF material has a chemical formula unit represented byCu₂(3,3′,5,5′-biphenyltetracarboxylate).
 2. The method of claim 1wherein selectivity of the MOF material for Xe from the gas mixture isabout 9 to about 11 over a pressure range of 0.01 to 1.0 MPa at 273 K.3. The method of claim 1 wherein the MOF material has 40% or more of thetotal pore volume in a pore size range of 0.45-0.75 nm.
 4. The method ofclaim 1 wherein the other noble gas is selected from the groupconsisting of Kr and Ar.
 5. The method of claim 1 wherein the MOFmaterial exhibits a selectivity for Xe from the gas mixture of about 9and 11 over a pressure range of 0.01 to 1.0 MPa at 273 K.